In heavy goods traffic, especially on roads with gradients, goods vehicles keep having serious accidents due to brake failures. In the U.S.A. alone there are more than 10 000 accidents of this kind each year.
In various countries, after longer gradients so-called run-out tracks are provided beside the roadway, onto which vehicles may be diverted in an emergency to run out on sand without causing damage.
In nearly all such accidents the vehicle is either overloaded, or travellig too fast, or its braking systems have been badly maintained. To analyze the causes of an accident, three variables must be known:
travelling speed PA1 axle loads PA1 braking power and its distribution between the individual wheels.
For the first of the three components--travelling speed--there are numerous measuring instruments available commercially today. For the second one likewise--axle load--there are static or so-called WIM (weight in motion) measuring systems capable of monitoring the traffic and, installed on flat stretches, detecting axle loads reliably.
For the third of the three components--braking--no measuring systems have become known till today.
Hitherto the brakes of vehicles have been tested from time to time on roller test stands in garages, though this resembles only remotely the actual conditions under overheating on gradients. The reduction of the braking power as the braking elements become progressively heated is known under the technical expression `fading`. Depending on the condition, i.e. the attention and maintenance given to the braking system, fading is more or less serious. Today no measuring system is able to detect this extremely important cause of countless accidents and thus help to combat these effectively.
The purpose of this invention is now to create for the first time a system capable of detecting the occurrence and magnitude of fading. The principal characteristic of fading is that it affects each braked wheel individually, so that the steerability of the vehicle is seriously influenced because wheel blocking may ensue at any time. Another consequence of fading is the malfunction of the ABS system now standard equipment, so that proper braking is no longer possible. The arrangement according to the invention for measuring this phenomenon consists of a measuring platform with a base plate and a cover plate, for fitting in the road surface and having between them at least two supporting points over the width of the cover plate in the direction of travel and a number of supporting points over the length of the platform transverse to the roadway. The new measuring platform is distinguished by piezoelectric crystal plates fitted in the supporting points and sustaining a thrust when the speed of a wheel changes and at the same time the axle load (F) of the wheel. The platform is fitted into the road surface so that the braking friction is measured as a shear force by the crystals in it. For this, the cover plate must be at least 50 mm (2") wide in the direction of travel, but it could also be the entire contact length of typically 250 mm with a vehicle tire. Through this contact surface of the cover plate the braking thrust is transmitted onto the measuring arrangement below, so that the signal magnitude is measured individually for each wheel.
Several of these thrust strips are disposed to advantage at defined intervals and with separate outputs for the left and right halves of the vehicles. Likewise advantageous is the alternate laying of WIM strips for detecting the axle weights. With these the vehicle speed can be detected at the same time.
So that the necessary heating of the braking parts occurs now, a brake system monitoring facility is fitted preferably after or on a gradient, so that the vehicles to be monitored have already covered a longer braking distance and the conditions exist for fading as described, and the different braking powers of the individual wheels are measured as shear forces. For this, measuring plates are required similar to those which measure the weight in motion (WIM), with the difference that these new measuring plates according to the invention measure shear forces or combinations of axle weights and shear forces.